The present invention relates to a process for filtering air in an apparatus referred to as a cyclone filter. It also relates to the apparatus, that is to say the cyclone filter and the filter cartridge which permit implementation of the process.
An example of this type of apparatus is described in the document FR-A-2,514,669 of the present inventor. These apparatus are particularly intended for extracting elements suspended in drawn air in premises for the production, treatment or storage of pulverulent products, or of certain industries generating the emission of wastes such as, for example, highly volatile splinters or sawdusts.
These apparatus consist of three principal elements, which are generally centered on one and the same vertical axis:
a cylindrical or conical upper body, in which the air to be de-dusted enters tangentially,
a settling chamber arranged underneath the upper body, this chamber being either in the form of a funnel or of cylindrical shape; it is equipped with means for picking up waste products in its lower part,
a filter cartridge consisting of filtering sleeves stretched vertically between the settling chamber and the cover of the apparatus where the filtered air is evacuated.
The air is purified in two stages. In the first stage, the heaviest dusts are separated by cyclonic centrifugation and fall directly into the settling chamber; and in the second stage, the residual dusts suspended in the air, which represent approximately 20% of the total, are retained in the filter cartridge and, in particular, on the vertical sleeves. These sleeves are generally made from a material of the needled-felt type. The quality of the filtering depends not only on the material used for the filter, but also on its condition and the speed of passage of the air. During its use, the filter progressively fills up, the pores become blocked, thereby creating a braking effect which is reflected at the air suction fan by a drop in flow rate. These apparatus are generally equipped, as described in the above-mentioned document, with an automatic unclogging device which makes it possible to regenerate filters. This unclogging takes place from the suction cover by means of jets of compressed air or the like which circulate countercurrentwise and which, for each filtering sleeve, repel the dusts which accumulate on the surface to the outside of the sleeves. The frequency of the unclogging operation is usually regulated by means of a cyclic programmer. The particles detached from the sleeves usually fall into the lower hopper.
Other apparatuses of this type are described, for example, in the document US-A-3,648,442. The filtering sleeves extend vertically in the body of the cyclone filter and are surrounded over a part of their height by a skirt whose function is to cover the inlet of charged air. These filtering sleeves are not, properly speaking, part of a specific cartridge associated with the body of the cyclone filter.
In such an apparatus, the lower part of the sleeves `silts up` fairly rapidly.
In the abovementioned document FR-A-2,514,669, the filter cartridge comprises a cylindrical casing which extends vertically over the entire height of the filtering sleeves, and this cartridge is open at both its ends. The lower end of the cartridge is located above the settling chamber, and is open to the outside of the sleeves. The upper end is being located in the cover of the apparatus, and is open to the inside of the sleeves. The charged air penetrates into the lower part of the filter cartridge, rises by means of suction into this cartridge around the sleeves, and, after having passed through the sleeves, is evacuated at the upper part of the cartridge referred to as the cover.
During unclogging, the particles detached from the surface of the sleeves usually fall by means of gravity; they drop more or less quickly as a function of the ascending speed of the air to be de-dusted which rises between the filtering sleeves. They are therefore stopped by the upward motion of the air in the cartridge between the sleeves. As the unclogging takes place sleeve by sleeve or by group of sleeves, the particles which are ejected from a sleeve and which are disturbed by the ascending speed of the air between the sleeves attach themselves to the adjacent sucking sleeve.
This same phenomenon is also found in the apparatus described in the above-mentioned document US-A-3,648,442. In this apparatus, the sleeves are encased only over a part of their height, and their lower part rapidly `silts up`. The ascending speed of the air in the lower part of the barrel very quickly becomes too great to allow the dusts detached from the surface of the sleeves during the unclogging operation to fall.
In fact, as the ascending speed of the air at the intake of the filter cartridge increases, the number of detached particles that can fall between the sleeves is reduced. Accordingly, the latter area clogs up still further because of the presence of detached particles which results in the air being forced to rise higher between the sleeves before passing through their wall, and therefore the inlet speed of the air in the cartridge is maintained over a greater height.